The invention relates to pressurized spray systems. Specifically, the invention relates to a spray system used to apply a polymer to the surface of a printing plate.
In general, the surface of a commercial printing plate is coated with reactive materials. The layer of reactive material applied to the plate is capable of being imaged (frequently the imaging process involves laser light) and is also capable of retaining the image on the plate. The image bearing plate may subsequently be used in a printing press to impart the image onto a printing surface. Recently, xe2x80x9cdirect-to-pressxe2x80x9d technology has permitted printing plates to be imaged directly on the printing press. The reactive materials used to coat printing plates are typically light or heat sensitive polymers, which are sprayed onto the printing plate through some sort of pressurized spray nozzle. Throughout this application, the word xe2x80x9cpolymerxe2x80x9d is used to describe the reactive material sprayed on a printing plate. However, the word xe2x80x9cpolymerxe2x80x9d should not be construed in a limiting sense, as the invention would apply to any material used to coat printing plates. In addition, the spray nozzle that is the subject of this invention may be used to temporarily spray other materials used in plate preparation, such as cleaning agents, stripping agents, water and other chemicals. For simplicity, use of the word xe2x80x9cpolymerxe2x80x9d should also be understood to incorporate these other materials temporarily sprayed on a printing plate surface.
Many types of spray nozzles exist in the prior art and they disclose various techniques for spraying different kinds of materials. U.S. Pat. No. 5,360,165 discloses a spray paint nozzle with a conical shroud, used to contain the sprayed paint, and a xe2x80x9crecirculation mechanismxe2x80x9d, used to reclaim leftover paint that does not adhere to the target surface. U.S. Pat. No. 5,441,201 discloses a liquid spray device for use in agricultural applications that is equipped with a shroud and uses hot air to break up conical sheets of sprayed liquid into droplets. U.S. Pat. No. 5,285,967 teaches a high velocity thermal spray gun for spraying high temperature melted powdered plastics, which also involves a shroud and a mechanism for cooling the melted powder down prior to reaching the target. U.S. Pat. No. 4,218,019 involves an xe2x80x9cair shroudxe2x80x9d, which contains the sprayed liquid and directs it toward the target. Finally, U.S. Pat. No. 5,057,342 discloses an apparatus for improving the feathering of the output from an xe2x80x9cairlessxe2x80x9d spray nozzle. While these prior art inventions teach techniques that may be generally applicable to spray nozzles, none of them address concerns particularly related to the application of polymer to the surface of printing plates.
In order to ensure the quality of the printed image the application of the polymers to a printing plate must be xe2x80x9ccompletexe2x80x9d (i.e. a coating on the entire imaging area of the plate) and xe2x80x9cuniformxe2x80x9d (i.e. a consistently even layer in all imaging areas of the plate). An additional consideration that is important to the spraying of polymers onto printing plates is the actual transfer efficiency of the spraying process. As the polymers used on printing plates are expensive, it is obviously beneficial to maximize the amount of polymer transferred from the nozzle to the plate and minimize the amount of wastage. These criteria of completeness, uniformity and transfer efficiency are most easily achieved when the polymer is sprayed in an atomized mist, in a fashion similar to commercially available xe2x80x9cspray-paintxe2x80x9d canisters. Unfortunately, the molecules of polymers used on commercial printing plates tend to become entangled with one another, making the polymer difficult to atomize, substantially reducing the effectiveness of conventional low-pressure spraying techniques. Accordingly, a spraying apparatus is required to obtain a complete, uniform coating of relatively entangled polymer on a printing plate with a high transfer efficiency.
In addition to the above requirements, the spraying of polymers onto printing plates involves a number of additional complications. Typically, the polymers used on printing plates are comprised of two or more reagents that must be mixed prior to spraying. The mixing process initiates a chemical reaction, similar to that of epoxy resin, which causes the polymer to cure and harden. Consequently, the mixing must be done immediately prior to spraying to avoid premature curing. In addition, any excess polymer that is mixed but not sprayed is wasted because it cures and is no longer sprayable. As such, an apparatus is required to mix the polymer""s constituent reagents immediately prior to spraying in a manner that will minimize the amount of polymer that is wasted by being mixed, but not sprayed.
Excess wastage is also a problem when applying polymer to a plate that is already mounted on the drum of a printing press. This process is common in today""s xe2x80x9cdirect-to-pressxe2x80x9d technology. Since part of the drum surface (referred to in this application as the xe2x80x9cplate-mounting gapxe2x80x9d or xe2x80x9cgapxe2x80x9d) is used to mount the plate and does not require a coating of polymer, any polymer sprayed into the plate-mounting gap is wasted. As such, an apparatus is required to minimize the amount of polymer sprayed into the plate-mounting gap.
A final consideration is the need for cleaning of the spray nozzle apparatus. If the polymer collects in the nozzle mechanism, it may cure and impair the functionality of the device. Consequently, an apparatus is required to facilitate the efficient cleaning of the spray apparatus.
The invention herein disclosed concerns an apparatus for a spray system operative to spray a substantially liquid polymer onto the surface of a printing plate. A fluid nozzle receives the polymer from the internal features of the spray system and ejects the polymer in a substantially liquid state. Surrounding the fluid nozzle, there is a conduit, which carries heated, high pressure air. The air heats the polymer, prior to its ejection from the fluid nozzle. The conduit also ejects the heated air, in such a manner that the air physically interacts with, and atomizes, the substantially liquid polymer, creating a mist of polymeric matter. The heating of the polymer by the air in the conduit makes it easier to atomize the polymer.
The apparatus also comprises a solid shroud surrounding the fluid nozzle and extending toward the printing plate. The shroud is equipped with at least one aperture attached to a vacuum source, such that the shroud, aperture and vacuum source act in combination to remove excess polymer that does not adhere to the printing plate. A fast shut-off valve, which controls the ejection of the polymer from the fluid nozzle, is located proximate to the fluid nozzle. In this manner, the amount of wasted polymer due to non-required ejection is minimized. The apparatus may also include a mixer operative to thoroughly and homogeneously mix the polymer from a number of constituent reagents. As with the shut-off valve, the mixer is located proximate to the fluid nozzle, so as to minimize the amount of the polymer, which is mixed, but not ejected.
Finally, the apparatus may comprise a cleaning mechanism. The cleaning system itself consists of a plurality of switches, which arrest the flow of the constituent reagents (if required) and permit at least one cleaning fluid to flow through (and simultaneously clean) the mixer, fast shut-off valve and fluid nozzle. The cleaning system also comprises a cleaning arm equipped with a source of vacuum suction. The cleaning arm moves, by either translation or rotation, between an active position (directly external to the fluid nozzle) and a non-intrusive position (out of the way, so as not to interfere with the ejected polymer). During cleaning, the cleaning arm is in the active position, directly external to the fluid nozzle. In this manner, the cleaning arm collects cleaning fluid, left-over polymer and any other materials ejected from the fluid nozzle.
Advantageously, the aperture located in the shroud may be further operative in combination with the vacuum source, to remove excess polymer (i.e. polymer that did not adhere to the target surface) from a vicinity of the fluid nozzle. In this manner, the excess polymer may be recycled.
Preferably, the fast shut-off valve may be located within the actual fluid nozzle.
Preferably, the mixer may further comprise a substantially cylindrical and hollow mixing column, which receives the constituent reagents from external reservoirs. The mixer may also include a substantially cylindrical mixing shaft, concentrically located within the mixing column. Finally, the mixer may also comprise a motor, which rotates the mixing shaft within the mixing column, so as to thoroughly and homogeneously mix the constituent reagents in a region between the exterior surface of the mixing shaft and the interior surface of the mixing column.
Advantageously, the exterior surface of the mixing shaft may be patterned, so as to improve the mixing process and to provide a suction force, which draws the constituent reagents into the mixing column.
The movement of the cleaning arm between the active position (i.e. directly external to the fluid nozzle) and the non-intrusive position may be accomplished by an external electro-mechanical switch or even by the vacuum suction source within the cleaning arm.
Advantageously, the plurality of switches and the cleaning arm, which comprise the cleaning mechanism, may be independently operative.
Preferably, the cleaning system may be further operative to assist in priming the spray system after cleaning. During a priming operation, the plurality of switches in the cleaning system may be operative to arrest a flow of the cleaning fluid (if required) and to permit a flow of the constituent reagents and polymer instead. The constituent reagents flow through the mixer and the polymer flows through both the fast shut off valve and the fluid nozzle. The cleaning arm may then be positioned directly external to the fluid nozzle and may be used to collect the substantially liquid polymer, any left-over cleaning fluid and any other materials that are ejected from the fluid nozzle, until the spray system is sufficiently primed.
Further advantages of the invention will become apparent when considering the drawings in conjunction with the detailed description.